1. Field of the Invention
The present invention relates to a planetary-gear-type multiple-step transmission interposed between a drive power source and drive wheels of a vehicle such as an automobile.
2. Discussion of Related Art
For vehicle, there is widely used a planetary-gear-type multiple-step transmission which is equipped with a plurality of planetary gear sets, as disclosed in JP-2002-206601A, JP-H08-105496A, JP-2000-199549A, JP-2000-266138A, JP-2001-82555A, JP-2002-227940A, JP-2002-295609A and JP-2956173B2. In such a planetary-gear-type multiple-step transmission, a plurality of predetermined speed ratios or operating positions (gear positions) are selectively established by connecting elements of the planetary gear sets through coupling devices such as clutches and brakes. For example, in a transmission disclosed in JP-2002-206601A, a total of twelve forward drive positions are established by using four planetary gear sets.
The planetary-gear-type multiple-step transmission is desired to be not only simple in construction and small in size and to provide a large number of operating positions and a wide range of speed ratio, but also to provide speed ratios that change in geometric progression or nearly geometric progression. In the multiple-step transmission disclosed in JP-2002-206601A, however, the speed ratios of the respective operating positions do not change in geometric progression, thereby resulting in a poor maneuverability in the transmission. In the transmission of JP-2002-206601A, for example, a ratio step between the speed ratio of the sixth-speed position and the speed ratio of the seventh-speed position is 1.050 (=1.391/1.325), which is considerably smaller than the other ratio steps between other adjacent speed positions (see FIG. 5 of the publication of Japanese Patent Application). That is, in this transmission, the ratio steps between adjacent speed positions considerably change, rather than being substantially uniform. Such a problem might be solved by increasing number of planetary gear sets to be incorporated in the transmission. However, the increase in the number of planetary gear sets leads to an increase in an overall length of the transmission, thereby possibly reducing a degree of freedom in installation of the transmission on a vehicle, while increasing number of coupling devices required for connecting elements of the planetary gear sets.